Critical mercury concentration in tropical soils: Impact on plants and soil biological attributes.

Mercury is a toxic element that becomes a problem when present at high concentrations in soils. Mercury toxicity in soils varies depending on chemical species, concentration, exposure routes, and organism vulnerability. There is little information regarding the toxicity of Hg in tropical soils, especially for establishing safe levels of this pollutant. The purpose of this study was to investigate Hg concentrations in two tropical soils and their effect on oats and common beans, as well as on soil biological attributes. The experiment was carried out in a greenhouse, following ISO 11.269-2 and OECD-208 guidelines. Oat and common bean were cultivated in a Typic Hapludox (TyHpx) and Rhodic Acrudox (RhAcx) contaminated with HgCl2 at the following concentrations: 0, 2.5, 5.0, 10.0, 20.0, 40.0, and 80.0 mg of Hg kg-1 of dry soil. The biological variables analyzed were seedling emergence, vegetative growth, chlorophyll content (SPAD index), gas exchange (photosynthetic rate, internal CO2 concentration, transpiration rate, and stomatal conductance), and Hg concentration and accumulation in shoot dry matter. Microbial biomass carbon, soil basal respiration, and metabolic quotient (qCO2) were also analyzed. Due to the sorptive characteristics of TyHpx, it had higher Hg concentrations than RhAcx. Mercury showed toxic effects on both oat and common bean species. However, common bean was affected only at concentrations higher than 20 mg kg-1. The microbial community showed high sensitivity to soil Hg concentrations, but external factors, such as the plant species cultivated, influenced the sensitivity of the community. The microbiota was most sensitive in pots with common bean, and this effect was more pronounced at low clay and low organic matter contents (TyHpx). In this study, the concentration of 0.36 mg kg-1 was critical for Hg in these soils, based on its deleterious effects on oat and common bean and on biological soil attributes.

Scaling like behaviour of resistivity observed in LaNiO3 thin films grown on SrTiO3 substrate by pulsed laser deposition.

We discuss the origin of the temperature dependence of resistivity ρ observed in highly oriented LaNiO3 thin films grown on SrTiO3 substrate by a pulsed laser deposition technique. All the experimental data are found to collapse into a single universal curve ρ(T, d) ∝ [T/T(sf)(d)]3/2 for the entire temperature interval (20K

Nodule development on the tropical legume Sesbania virgata under flooded and non-flooded conditions.

The interaction between the Brazilian pioneer legume Sesbania virgata and its microsymbiont Azorhizobium doebereinerae leads to the formation of nitrogen-fixing nodules on roots that grow either in well-aerated soils or in wetlands. We studied the initiation and development of nodules under these alternative conditions. To this end, light and fluorescence microscopy were used to follow the bacterial colonisation and invasion into the host and, by means of transmission electron microscopy, we could observe the intracellular entry. Under hydroponic conditions, intercellular invasion took place at lateral root bases and mature nodules were round and determinate. However, on roots grown in vermiculite that allows aerated growth, bacteria also entered via root hair invasion and nodules were both of the determinate and indeterminate type. Such versatility in entry and developmental plasticity, as previously described in Sesbania rostrata, enables efficient nodulation in both dry and wet environments and are an important adaptive feature of this group of semi-tropical plants that grow in temporarily flooded habitats.

Temperature oscillations of magnetization observed in nanofluid ferromagnetic graphite.

We report on unusual magnetic properties observed for nanofluid room temperature ferromagnetic graphite (with an average particle size of [Formula: see text] nm). More precisely, the measured magnetization exhibits a low temperature anomaly (attributed to the manifestation of finite size effects below the quantum temperature [Formula: see text]) as well as pronounced temperature oscillations above T = 50 K (attributed to manifestation of the hard-sphere type of pair correlations between ferromagnetic particles in the nanofluid).

Superconducting and magnetic behaviour of niobium doped RuSr(2)Gd(1.5)Ce(0.5)Cu(2)O(10-δ).

Polycrystalline samples of Ru(1-x)Nb(x)Sr(2)Gd(1.5)Ce(0.5)Cu(2)O(10-δ), 0≤x≤0.5, have been synthesized and structurally characterized by x-ray diffraction (XRD). Resistivity, magnetization and AC susceptibility measurements have been done and analysed considering a phase separation scenario. A strong suppression of the cluster glass (CG) transition associated with niobium doping was identified. In fact, the CG phase was not present in samples for x≥0.2, leading to changes in the magnetic hysteresis loops measured at low temperatures. These hysteresis loops can be explained as a result of the contribution of two distinct magnetic phases: the canted AFM phase and embedded Ru(4+)-rich clusters which order as a CG in low temperatures. Interestingly, the significant changes in the magnetic response of the material do affect the superconducting transition temperature T(c). It was found that both T(c) and the superconducting fraction are reduced in samples which present the spin glass phase. Therefore, our results point to some coupling between magnetism and superconductivity in this ruthenocuprate family, the presence of the magnetic moment being deleterious for the superconductivity.

Survey of mycobacteriology laboratory practices in an urban area with hyperendemic pulmonary tuberculosis.

SETTING: Rio de Janeiro, Brazil, where 10210 cases of tuberculosis (TB) were reported in 1997, 86.2% of them with pulmonary TB. OBJECTIVE: To assess laboratory resources, practices, biosafety measures and training needs relative to the volume of work required for the TB control program and implementation of directly observed therapy, short course (DOTS). DESIGN: A cross-sectional survey of laboratories that receive funds from the public sector and work with mycobacteria, using a structured questionnaire and onsite visits to collect data. The main outcome measure of interest was processing > or = 20 specimens per week. RESULTS: More than half (56.5%) of the laboratories reported performing < 20 specimens per week, a level at which it is difficult to maintain proficiency in mycobacteriologic techniques. The demand for sputum smear microscopy was not met. Working conditions such as shared laboratory and air space, inadequate ventilation, accidents with biological specimens, and inadequate disposal of biological waste present risks of TB transmission to laboratory workers and other staff. CONCLUSION: Training and supervising laboratory workers in good technique and biosafety practices and providing the necessary organization, resources and working conditions will strengthen TB control and facilitate implementation of DOTS. Several simple interventions are proposed.

Biodiversity of rhizobia isolated from a wide range of forest legumes in Brazil.

Tropical forests have a high diversity of plant species; are they associated with a correspondingly rich microbial flora? We addressed this question by examining the symbiotic rhizobium bacteria that nodulate a diverse pool of forest legume species in Brazil. The 44 strains studied had been isolated from 29 legume tree species representing 13 tribes including all three subfamilies of the Leguminosae, and were chosen to represent major groups from a larger sample that had previously been characterized by SDS-PAGE of total proteins. Partial 16S rRNA gene sequence was determined, corresponding to positions 44-303 in the Escherichia coli sequence. Fifteen sequences were found, including six novel ones. However, all but one of them could be assigned to a genus because they grouped closely with sequences from previously described rhizobial species. Fast-growing strains had sequences similar to Rhizobium spp., Sinorhizobium spp. or Mesorhizobium spp., while the slow-growing strains had sequences similar to Bradyrhizobium spp. One strain with an intermediate growth rate had a unique sequence which indicated that the strain might belong to the genus Azorhizobium. Although the strains showed a variety of sequences, it was surprising that these strains isolated from taxonomically very diverse host plants in previously unexplored environments were mostly very similar to strains described previously, largely from agricultural systems.